AP Photo/Paul Sancya
Donald Trump.
"Ted Cruz, he's seriously - he's got a mental problem. Ted Cruz is a liar. He's got a problem, that he's a liar. I've never seen anybody [lie] to that extent," Trump said.
Trump has repeatedly stated his disgust for Cruz in recent days but ratcheted up his attacks over the past weekend.
On Monday, Trump even threatened to sue Cruz over his eligibility to be president if he didn't retract "lies" about Trump's record. Cruz, one of Trump's top GOP rivals, has argued that voters should be concerned about the Republican front-runner's past support for Democrats and left-leaning policies.
"Cruz is the one that does the personal attacks on me. There's never been any politician - or almost anybody in my life that I've seen - that lies so much," Trump said on Fox Business.
Trump added:
He will say I'm against the Second Amendment. He will say I'm totally in favor of Obamacare, which is ridiculous. These are things that are staples of what I'm about. I'm totally in favor of the Second Amendment. … Obamacare, I'm absolutely totally going to repeal it and replace it. And Ted Cruz says, "Oh, he's a big fan of Obamacare." He is a disgrace. He is a liar.
For his part, Cruz dismissed Trump's accusations as unfounded. He has sought to group Trump with Sen. Marco Rubio (R-Florida), another presidential candidate who also accuses Cruz of lying about his record.
"It really is remarkable," Cruz said Monday when asked about Trump's lawsuit threat on Fox News.
"And it's quite odd that both Marco Rubio and Donald Trump respond the same way," Cruz added. "Which is that when anyone points to their actual records, they get very upset and begin screaming, 'Liar, liar, liar.'"